Display devices have been in production for a wide range of electronic applications, such as flat screen televisions (TV), flat monitors, mobile phone, MP3 players, electronic book or eBook readers, and personal digital assistants (PDAs) and the like. The display devices are designed for producing a desired image by applying an electric field to a liquid crystal that fills a gap between two substrates and has an anisotropic dielectric constant that controls the intensity of the dielectric field. By adjusting the amount of light transmitted through the substrates, the light and image intensity, image quality, and/or power consumption may be efficiently controlled.
Thin film transistors (TFTs) for flat panel displays benefit from a lower processing temperature (e.g., 350° C. or below) so that alternative substrates that are lighter and less expensive than the presently used substrate or glass can be used. Various display devices, such as active matrix liquid crystal display (AMLCD) or an active matrix organic light emitting diodes (AMOLED), can be employed as light sources for display devices which use touch screen panels. Amorphous oxide semiconductors (AOS), transparent amorphous oxide semiconductor (TAOS) or metal oxide materials are fast emerging as replacement materials for TFTs that provides higher performance than glass be improving the device's electrical performance and are processable at lower temperatures. Examples of AOS, transparent amorphous oxide semiconductor (TAOS) or metal oxide materials that are being considered as replacements for TFTs include Indium Gallium Zinc Oxide (IGZO), a-IGZO (amorphous gallium indium zinc oxide), Indium Tin Zinc Oxide (ITZO), Aluminum Indium Oxide (AlInOx), Zinc Tin Oxide (ZTO), Zinc Oxynitride (ZnON), Magnesium Zinc Oxide, zinc oxide (ZnO) and variations thereof. Despite their advantages over traditional materials, these materials have a temperature processing limitation of about 350° C. or less. Further, these films may be deposited onto plastic substrates which lower their temperature processing limitation to about 200° C. Additionally, certain AOS, TAOS, or metal oxide materials may be damaged by the presence of hydrogen atoms in adjacent passivation, gate insulating layers, or both by reacting with the transparent amorphous oxide semiconductor (TAOS) or metal oxide materials, thereby resulting in current leakage or other types of device failure.
The reference “Influence of Passivation Layers on Characteristics of a-InGaZnO Thin-Film Transistors”, Liu et al., Electron Device Letters, IEEE, Vol. 32(2), (20110, pp. 161-63 (“Liu et al.”), investigated the effect of deposition conditions of a dual passivation layer consisting of silicon oxide and silicon nitride atop on the threshold voltage (Vt) of the a-InGaZnO TFT. The test structure used in Liu et al. consisted of a p-type silicon wafer which had a silicon substrate that served as the gate electrode, a 200 nanometer (nm) thick thermally grown silicon dioxide layer which acted as the gate insulator layer, a 45 nm thick source/drawn (Al) electrodes adjacent a 50 nm thick a-IGZO channel layer. The Al electrodes and a-IGZO layer was topped with a dual passivation layer consisting of a 30 nm silicon oxide layer and a 180 nm thick silicon nitride layer. The silicon oxide and silicon nitride films were deposited by plasma enhanced chemical vapor deposition (PECVD) at 200° C. using SiH4/N2O/N2 and 250° C. using SiH4/NH3/N2, respectively. The threshold voltage (VT) of the TFTs shifted markedly as a result of the mechanical stress induced by the passivation layers above. By adjusting the deposition parameters of the silicon nitride top layer during the passivation process, the performance of the TFTs can be modulated. The optimized a-InGaZnO TFTs after dual passivation exhibited the following characteristics: a field-effect mobility of 11.35 cm2/V·s, a threshold voltage of 2.86 V, a subthreshold swing of 0.5V, and an on-off ratio of 108.
The reference “Impact of Hydrogenation of ZnO TFTs by Plasma-Deposited Silicon Nitride Gate Dielectric”, Remashan et al., IEEE Transactions on Electronic Devices, Vol. 55, No. 10 (October 2008), pp. 2736-43, describes the effects of depositing by PECVD a silicon nitride layer having variable refractive indices for use as a gate dielectric layer on a zinc oxide (ZnO) TFT with a bottom gate configuration. The authors stated that hydrogenation is one of the methods in which performance of ZnO TFTs can be improved because hydrogen acts as a defect passivator and a shallow n-type dopant in ZnO materials. In Remashan et al., the four silicon nitride films were deposited via PECVD at a pressure of 650 mTorr, temperature of 300° C., and power of 30 W but using different molar ratios of silane relative to ammonia and nitrogen to provide silicon nitride films having different refractive indices (e.g., 2.39, 2.26, 1.92, and 1.80) and dielectric constants (7.9, 8.4, 6.7, and 6.1). The authors found that the amongst all of the TFTs, the device having the highest refractive index silicon nitride film or SiN_2.39 exhibited the best performance in terms of field-effect mobility, subthreshold slope, and maximum interface state density. An analysis of the secondary ion mass spectroscopy (SIMS) data showed that the amount of hydrogen present at the ZnO/insulator interface and in the ZnO channels for the TFT structures using a SiN_2.39 was much higher than those structures using a SiN_1.80. Therefore, the authors have concluded that the enhanced performance of the TFTs using the SiN_2.39 films is attributed to the incorporation of hydrogen into the ZnO channel and ZnO/insulator interface from the SiN_2.39.
The reference “Circuits Using Uniform TFTs Based on Amorphous In—Ga—Zn—O”, Ryo Hayashi et al., Journal of the Society for Information Display, Vol. 15(11), 2007, pp. 915-92 discloses high-performance and excellent-uniformity thin-film transistors (TFTs) having bottom-gate structures fabricated using an amorphous indium-gallium-zinc-oxide (IGZO) film and an amorphous-silicon dioxide film as the channel layer and the gate insulator layer, respectively. All of the 94 TFTs fabricated with an area 1 cm2 show almost identical transfer characteristics: the average saturation mobility is 14.6 cm2/(V-sec) with a small standard deviation of 0.11 cm2/(V-sec). A five-stage ring-oscillator composed of these TFTs operates at 410 kHz at an input voltage of 18 V. Pixel-driving circuits based on these TFTs are also fabricated with organic light-emitting diodes (OLED) which are monolithically integrated on the same substrate. It was demonstrated that light emission from the OLED cells can be switched and modulated by a 120-Hz ac signal input. Amorphous-IGZO-based TFTs are prominent candidates for building blocks of large-area OLED-display electronics.
The reference, “Stability and High-Frequency Operation of Amorphous In—Ga—Zn—O Thin-Film Transistors with Various Passivation Layers”, Kenji Nomura et al., Thin Solid Films, doi:10.1016/j.tsf.2011.10.068 (2011), investigated the stability of amorphous In—Ga—Zn—O (a-IGZO) thin-film transistors (TFTs) focusing on the effects of passivation layer materials (Y2O3, Al2O3, HfO2, and SiO2) and thermal annealing. Positive bias constant current stress (CCS), negative bias stress without light illumination (NBS), and negative bias light illumination stress (NBLS) were examined. It was found that Y2O3 was the best passivation layer material in this study in terms of all the stability tests if the channel was annealed prior to the passivation formation (post-deposition annealing) and the passivation layer was annealed at 250° C. (post-fabrication annealing). Post-fabrication thermal annealing of the Y2O3 passivation layer produced very stable TFTs against the CCS and NBS stresses and eliminated sub gap photoresponse up to the photon energy of 2.9 eV. Even for NBLS with 2.7 eV photons, the threshold voltage shift is suppressed well to −4.4 V after 3 hours of testing. These results provide the following information; (i) passivation removes the surface deep subgap defects in a-IGZO and eliminates the subgap photoresponse, but (ii) the bulk defects in a-IGZO should be removed prior to the passivation process. The Y2O3-passivated TFT is not only stable for these stress conditions, but is also compatible with high-frequency operation with the current gain cut-off frequency of 91 kHz, which is consistent with the static characteristics.
US Publ. No. 2012/045904 (“the '904 Publ.”) discloses methods of forming a hydrogen free silicon containing layer in TFT devices. The hydrogen free silicon containing layer may be used as a passivation layer, a gate dielectric layer, an etch stop layer, or other suitable layers in TFT devices, photodiodes, semiconductor diode, light-emitting diode (LED), or organic light-emitting diode (OLED), or other suitable display applications. In one embodiment, a method for forming a hydrogen free silicon containing layer in a thin film transistor includes supplying a gas mixture comprising a hydrogen free silicon containing gas and a reacting gas into a plasma enhanced chemical vapor deposition chamber, wherein the hydrogen free silicon containing gas is selected from a group consisting of SiF4, SiCl4, Si2Cl6, and forming a hydrogen free silicon containing layer on the substrate in the presence of the gas mixture.
US Publ. No. 2010/059756 (“'the 756 Publ.”) disclose a thin film transistor (TFT). The TFT may include an intermediate layer between a channel and a source and drain. An increased off current which may occur to a drain area of the TFT is reduced due to the intermediate layer which is formed of amorphous silicon (a-Si), poly-Si, germanium (Ge), or silicon-germanium (SiGe).
Therefore, there is a need for a display device and method to manufacture same that provides one or more of the following advantages: good electrical properties meaning that it retains its semiconductive nature after processing; low processing temperatures (e.g., 350° C. or less) reduced hydrogen content; improved electrical performance; and long term stability.